There are many different types of intervention cables for working in a well. In wireline operation in a well one may use a smooth, thin cable with Ø about 3 mm with central electrical conductors and a smooth steel mantle. Other wireline cables may have central electrical conductors and a braided or twisted wire mantle. Wireline cables may also have a fibre core in order to increase the ultimate strength.
Rigid intervention cables of composite material for use in petroleum wells have such a high flexural rigidity that they may be fed into a well by rodding and is thus often called a rod. such intervention cables often have a diameter of about 10 mm and are resiliently pliable about a smallest allowable bending radius of about 2-4 m without being plastically deformed. The diameter of such rigid intervention cables may for practical applications be embodied in 8 mm, 10 mm, 12 mm, and up to 15 mm, with progressively increasing smallest allowable bending radius. The resilient intervention cable usually comprises one or more electrical and/or optical conductors in a central cable portion of diameter about 2-6 mm, and a composite fibre mantle outside on the core, filling in to the diameter of 10 mm. The composite fibre mantle may, in some embodiments have longitudinally directed carbon fibres as main component and a matrix of thermoplastics or cured plastics
Upon rodding of such a rigid, resilient intervention cable from a drum on a petroleum installation at a petroleum well, via a well injector, it is imperative that it is not bent further than its smallest allowable bending radius. In this way a permanent curvature of the cable is avoided, and it essentially straightens itself completely out when it is released. Upon hauling of the intervention cable the same applies. It is usual in the background art to let an intervention cable run as an air span between a gooseneck on a well injector an intevention cable drum, and let the downward deflection control the amount of slack of the intervention and thus the length which at any instant resides between the gooseneck and the drum. This provides an uneven tension to the intervention cable when it enters the drum and is undesired. This also provides an uneven upper force from the intervention cable when it runs between the gooseneck and the injector, a uneven so-called back tension, and is also undesired. A freely suspended intervention which heaves irregularly above deck is also undesired with regard to the deck crew's safety and requires free space between the units, and thus much wasted space.
A free air span between the well injector and the drum also limits the options for utilizing the intermediate deck area for other activities. It is thus desirable to let the rigid intervention cable run through a fixed path between the injectors and the drum, through at least so-called “bending restrictor” at either ends of a possible rigid pipe path between those. A bending restrictor may in practice be an articulated pipe body which may be bent but wherein each articulation may only be bent so as for said intervention cable to be locally plied to a bending radius which is larger than or equal to the smallest allowable bending radius for the rigid intevention cable. It is essential that the intervention cable bending restrictor, which comprises a series of links, is non-compressible along its path, and a that it also may not extended to any mentionable degree, and that it offers a particular resistance against being further bent than said smallest allowable bending radius for a given intervention cable.
The applicant itself has a patent application on a bending restrictor: WO2011/096820 to Helvik, wherein said bending restrictor comprises short pipe sections with a spherical sector at either end, wherein two and two pipe sections are joined using a split sleeve with spherical sector shaped seats at either and, and whereupon is arranged a locking ring at either ends of said split sleeve. Either pipe section's end comprises a ring-shaped collar which forms a limit of each spherical sector towards the pipe section's straight portion. At the same time the ring shaped collar forms a shoulder which forms a limitation to how far the closed collar's end may be pivoted about the spherical sector. In this way a series of such pipe sections and closed collars form a bending restrictor which assembled form a tubular body. This tubular body is not much compressible in its longitudinal direction and little extendable, and for that matter works as a bending restrictor.
A problem related to the above mentioned bending restrictor of the background art of WO2011/096820 is, that upon bending until the collar meets the end of the sleeve in one peripheral point, and thus prevents further bending about the spherical articulation, large mutual point forces arise between those and deformation is initiated, either in the collar or in the split sleeve's end, which is just split and does not very well withstand hoop forces The bending restrictor according to WO2011/096820 thus, due to the point contact against the collar, a not well defined end point and thus a somewhat undefined smallest allowable bending radius. Further, it comprises many components, and because each link is rather short, several manipulations are required to assemble a desired length of bending restrictor. The spherical sectors with the ring collar against the pipe sections' ends and the resulting mutual point contact between the pipe sections end and the ring collar incurs large bending moments which are taken up between the pipe sections' spherical sector shaped end and the spherical sector seat, a bending moment which has a short arm, and may deform the split spherical sector seat to an undesired degree.